Radio frequency heater and control therefor

ABSTRACT

An RF heater having an oscillator including a tank circuit and a feedback system for switching fixed capacitance values into the tank circuit in response to load changes. Also disclosed are a longer length, tapered work coil of variable pitch, means for controlling sections of the work coil independently and a form of oven including the coil for continuous and discontinuous heating of untreated or treated workpieces.

D United States Patent 1191 1111 1 3,870,847 Shields Mar. 11 1975 RADIO FREQUENCY HEATER AND 2,441,435 5/1942 Mittlemann 219/1077 x 2,452,l97 I0 194 Kennedy 2l9/l().75 CONTROL THEREFOR 2,548,246 4/l95l Walstrom 2l9/l(].77 75 Inventor: John P. Shields, Cl velan Ohi 2,757,266 7/1956 Mauwauing 219/1049 Assign: Allegheny Plastics, Inc. Commons, 3,185,8ll 5/l965 Kasper et al 2l9/l0.77

Primary E.\'aminer-Bruce A. Reynolds [22] Filed! May 29, 1973 Attorney, Agent, or FirmWebb, Burden, Robinson & 21 Appl. No.; 365,038 Webb 57 ABSTRACT [52] U.S. Cl 2l9/l0.77, 2l9/l0.67, 2l9/l0.79 I 1 511 1m. 01. H05b 5/06 RF heater [58] Field of Search 219/) 57 1O 75 1O 77 cu1t and a feedback system for switching fixed capaci- 79 6 tance values into the tank circuit in response to loud changes. Also disclosed are a longer length, tapered k coil of variable pitch, means for controlling sec- [56] References Cited f t1ons of the work COll independently and a form of UNITED STATES PATENTS oven including the coil for continuous and discontinu- I Northrup X ous heating of untreated or treated workpieces 2.l76,l03 10/1939 Rouay 2l9/l0.67 X 2,4l6,l72 2/1947 Gregory et al 2l9/l0.77 9 Claims, 4 Drawing Figures 4 TIME DELAY :1 RELAY L6 q d 9 J 1 TIME DELAY AMPLlFlER RELAY 4 AMPLIFIER l PLATE AI SUPPLY mwzmEwzqE. mEjm sum 2 or 3 PATENTED NARI I975 RADIO FREQUENCY HEATER AND CONTROL THEREFOR This invention relates to an RF heater and more particularly to a means for and method of controlling the operating characteristics of such a heater in order to heat both ferrous and non-ferrous materials.

More specifically, the invention provides a means for and method of sensing the type of material to be heated and automatically adjusts the characteristics of the oscillator to provide themost efficient material heating.

The invention is applicable to continuous and discontinuous heating of workpieces. It isuseful for-heating both treated and untreated workpieces. By treated is meant. that the workpiece has a coating of material such as resin or the like.

BACKGROUND OF THE INVENTION In the conventional radio-frequency heater, an oscillator, usually an electron tube-type oscillator is used to generate the required radio frequency energy. This radio frequency energy is applied to the material to be heated by means of a work or load coil. In the conventional arrangement, the work coil consists of a few turns of copper tubing which surround the material to be heated. In instances where a large length to diameter coil is desired, it is conventional practice to employ a rather complex impedance matching system to match the long work coil to the radio frequency oscillator. While these external matching circuits permit the use of a large length to diameter work coil, they add complexity and cost to the system.

In a radio frequency heating oscillator the amount of tank circuit loading is determined primarily by the type of material placed within the tank or work coil; Ferrous materials, such as iron or steel, due to their high permeability offer the maximum loading, while non-ferrous materials, such as brass or aluminum, offer little loadmg.

It is possible to achieve efficient oscillator loading of non-ferrous materials by employing a high L-C ratio in the tank circuit, thus having the effect of greatly increasing the circulating current in the tank circuit. This increased circulating tank circuit current provides sufficient varying magnectic flux density to provide heating of non-ferrous materials. In a sense, the high capacitance tank circuit provides forced loading".

While a high capacitance tank circuit will provide efficient heating of non-ferrous materials, it is not suited for ferrous material heating as heating efficiency will be degraded. Thus, it is apparent that two separate tank circuit configurations must be employed when heating ferrous and non-ferrous materials. It is the purpose of this invention to provide means for automatically sensing the specific type of material to be heated and then to automatically effect the proper tank circuit configuration.

BRIEF DESCRIPTION OF THE INVENTION Briefly, the invention comprises an RF heater having an oscillator including a tank circuit and a feedback system for switching fixed capacitance values into the tank circuit in response to load changes depending upon the materials to be heated. I prefer to use an electron tube-type oscillator, e.g., a vacuum tube; however solid state devices, e.g., transistors, may be used.

In considering the invention, the work coil and radio frequency oscillator may be viewed as an integral unit wherein the oscillator circuit values are selected for use with a single coil. Should a different work coil length be used, or should a'coil designed for use with different loads be required, the oscillator circuit values may be changed to obtain optimum operating efficiency with the particular coil.

I have also disclosed a longer length tapered work coil of variable pitch, and means for controlling sections of the work coil independently. Finally, I have disclosed a form of an RF oven for heating materials including such a coil.

A further understanding of the invention will be obtained from a review of the following specification when taken with the accompanying drawings in which:

FIG. I is a block diagram of the elements of the invention;

FIG. 2 is a circuit diagram of a preferred embodiment of the invention;

FIG. 3 is a diagrammatic view of a work coil according to the invention; and

FIG. 4 is a perspective view of an RF oven in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION A tank circuit 1 of a conventional radio frequency heating oscillator 2 includes a work coil 3 for heating materials either individually or continuously. According to the invention, a control circuit having a feedback loop is interposed between the plate supply 4 of the oscillator and the tank circuit for optimally loading the tank circuit in response to the characteristics of the material to be heated.

The control circuit comprises a pair of capacitors 5, 6 having fixed values which are engageable in the tank circuit with the work coil 2. Means are provided for selecting the proper capacitance ratio in the tank circuit for ferrous or non-ferrous materials to be heated comprising means for sensing a change in operating or plate current and responsive to such change for opening the plate supply, means for switching the capacitance values while the supply is de-energized, and means for closing the plate supply after the proper capacitance is engaged in the tank circuit.

Preferably, the means for sensing a change in the plate current comprises an amplifier 7 which is electrically connected to the plate supply and to a first time delay switch 8, which in turn is electrically connected to the plate supply. A switch 9 is electrically connected to the time delay 8 for switching the capacitance in the tank circuit. Amplifier 7 is also electricallyconnected to a second time delay relay 10, which in turn is connected to the plate supply for closing the plate supply after selection of the proper capacitance ratio in the circuit. An undercurrent amplifier 11 may also be provided between the input to the amplifier 7 and the output of the time delay relay 8 to the plate supply for unlatching the relays in response to a plate current output from the oscillator which is lower than a predetermined current value, as when a workpiece is removed from the work coil.

The ratio of the capacitance to the inductance in the tank circuit is varied in accordance with the characteristics of the material being passed individually or continuously through the work coil. A ferrous material has a low capacitance to inductance ratio; thus it is a high lected in accordance with the frequency used. For example, where the frequency is 455 khz, capacitor 5 has a value of 0.00l mf and capacitor 6 has a value of 0.01 mf. When a ferrous material is heated, the capacitor 5 is used; when non-ferrous material enters the coil for heating, the circuit automatically switches capacitor 6 into the circuit effectively'presenting the oscillator with a larger load which results in a greater circulating current in the tank circuit and provides sufficient varying magnetic flux density to heat the non-ferrous material.

A change in the plate current is reflected as a change in Al passing from the plate supply 4 to amplifier 7. When the component of AI which passes from amplifier 7 to the time delay relay 8 is sensed by the relay 8 it opens the plate current and holds it open for av certain period, for example, 2 seconds. While the platecurrent is de-energized, capacitor 6 is removed from the tank circuit. Once the capacitor 6 is removed to present a different capacitance in the tank circuit, time delay relay 10 is actuated closing the plate supply and permitting current to again flow through the circuit.

To avoid arcing during switching of capacitor 6 in response to changes in materials being heated by the work coil, the time delay relay 10 is programmed, e.g., 4 seconds, to close the plate supply only after time delay relay 8 (at 2 sec.) has removed capacitor 6 from the circuit.

The basic operation of the invention can be understood by reference to the block diagram in FIG. 1. If it be assumedthat a ferrous material is placed within the work coil 3, it is apparent that the plate current of the oscillator 2 will be increased to a predetermined value after a plate supply voltage is applied. A portion of the plate current is then sampled and applied as a correction signal to amplifier 7. The output for amplifier 7 is applied to the time delay relay 8 which switches the correct value of capacitance into the tank circuit 1 to obtain proper oscillator loading for ferrous materials. The output from amplifier 7 is also applied to the time delay relay 10 which momentarily interrupts the plate voltage to the oscillator. Thus, at the instant the tank circuit capacitance is-being switched, plate voltage is removed. This also removes any voltage from across. the capacitor switching relay contacts at the instant of switching.

The time delay portion of the time delay relay circuit also allows time for the plate voltage to be removed from the oscillator before the capacitor switching relay is energized.

If it now be assumed that a non-ferrous piece of material is inserted in the tank coil, this will result in a drop in oscillator plate current. The resulting change and correction signal will be-sensed by amplifier 7 and applied to the time delay relay 8. However, since there is insufficient current to trigger time delay relay 8, switch 9 is closed placing capacitor 6 into the circuit.

Latching circuitry is provided so that once the proper circuit capacitance has been'selected, there will be no further need to rely on the plate current change until the workpieces within the tank coil have been removed and a new workpiece inserted. This is essential to prevent hunting of the feedback circuit due to slight 4. changes in plate currents which may occur'as the workpiece is being'heated.

When theworkpiece is removed from the tank coil, the latching circuits are opened by the output signal from amplifier 11.

In FIG. 1, I have diagrammatically illustrated my preferred control circuit comprising a Hartley oscillator including a triode 20. The circuit components may be best described andunderstood by reference to FIG. 2 and to its operation.

If a ferrous material is to be heated, plate supply voltage (for example 4,000 V) is applied at 21 and the plate transformer 22 has current (i.e., 20 amps). When a ferrous workpiece 23 is advanced to the work coil 3 there is an increase in plate current (i.e., to 25 amps) and the increase is reflected in the secondary 25 of a current transformer 24. The increase is rectified by the diode 26 to develop a positive voltage across a calibration potentiometer 27. The positive voltage is applied to the base of transistor 28 and is of sufficient magnitude to increase the collector current of the transistor to a value sufficient to energize relay 29 and'close ganged contact sets 30, 31 and 32. The closing of set 30 causes current to be applied to thermal time delay relay 33. Simultaneously, closing of contact set 31 applies operating current to relay 34 causing its contacts to open removing current from the primary of the plate transformer 22.

After a brief time delay (for example, 2 seconds) the thermally responsive contacts of'relay 33, which are normally open, close energizing relay 35, opening its contacts, and thereby removing capacitor 6 from the tank circuit 1.

When the contact set 31 was closed, power was applied to the thermal time delay relay 36 whose contacts are normally closed. After a time delay (approximately 4 to 6 seconds), the contacts open removing current from relay 34, the contacts-of which close restoring current to the primary of the plate transformer 22.

After the workpiece is removed from the work coil, the bias voltage of transistor 38, which was latched as soon as the work-piece entered the work coil, drops to a value sufficient to de-energize relay 39, closing its contacts to apply current to latch coil 40 which releases relay 29.

Assume that a non-ferrous material-is subsequently to be heated. When the non-ferrous material is advanced to the work coil 3, the plate current of the plate transformer 22 drops causing a drop in the base current of transistor 28 to a value such that relay 29 is deenergized and there is insufficient current to activate thermal relay 33. Since relay 35 is responsive to relay 33 it is also de-energized, its contacts are closed and both capacitors 5 and 6 remain in the tank circuit.

A preferred work coil for use in my invention is shown diagrammatically in FIG. 3 and comprises a helically wound strip 40 of a flat conductive material, such as brass. Use of flat highly conductive strip permits the use of forced air cooling as contrasted to water cooling used in conventional tubular copper windings employed in present coils. This also represents savings in cost and complexity.

The work coil winding is tapered from one end 41 to the other end 42. This permits the operator to control the amount of magnetic flux density applied to the material to be heated. The tumor pitch can also be designed to suit the desired heating application. Finally,

as also shown in FIG. 3, a novel method of power control can be obtained by the useof a relay contact 43 which is arranged to switch a portion of the winding in response to a control signal derived from a temperature controller 15. Thus, a first portion 45 of the coil winding can be energized at all times to serve as a preheater for the work while a second portion 46 of the coil can be indefinitely cycled to maintain the desired work temperature.

My preferred oven 47 includes a sufficiently long, heating zone. In FIG. 4, the oven is shown as having a square cross-section; however, and cross-sectional configuration will suffice. The inside of the oven is covered with a heat-resistant material 48, such as asbestos. A flat brass work coil 49 is helically applied to the nonconductive material layer 50, which is preferably polypropylene, and is appropriately electrically connected to the balance of the tank circuit in a well-known manner. A second non-conductive material layer 51, such as polypropylene, is placed around the outside of and spaced from the layer 50 to provide an air space 52 for the work coil. Cooling air may be circulated through the air space to dissipate the heat from the coil. The oven is enclosed by a metal sheath 53 which confines the radio frequency rays to the oven to protect an operator from any health hazard. A gap 54 is provided in the metal sheath to eliminate conduction of electricity through the sheath. I have designed a satisfactory oven approximately 6 inches by 6 inches by 2 feet long using as the work coil a flat brass strip (three-fourths inches wide approx.) having 20 turns.

The invention is applicable to an individual or batch process in which a single workpiece to be heated is placed within the work coil or to a continuous process in which different workpieces are moved continuously through the oven. A suitable means for continuously heating individual pieces is a belt-type conveyor 55 (FIG. 4) in which a non-conductive belting material is provided upon which the workpieces to be heated are moved. The belt is motor driven and travels over rolls 56 at each end of the oven. I

My oven may also be used for heating sheet material, which passes through the oven in a manner similar to a belt-type conveyor. In this case, the material is taken up in coil form from an idler capstan at the entrance end and pulled through the oven on a continuous basis by a driven capstan at the exit end of the oven. The same principles apply to the heating of the material by a continuous process as in the case of a batch type process and the same electrical circuitry can be used. The process is applicable to heating untreated workpieces such as annealing or for processing treated workpieces, i.e., those which have been coated, for example with vinyl epoxy, plastic, Teflon, rubber or other materials wherein the substrate is heated.

The invention provides significant production advantages in terms of cycle time, costs and manpower.

Having described certain preferred embodiments of the invention, it is to be understood that it is limited only by the scope of the appended claims.

I claim:

1. An RF heater comprising:

A. a power source;

B. an oscillator having a tank circuit including at least two capacitors and a work coil;

C. means for sensing a change in operating current of the oscillator caused by a workpiece entering the coil;

D. means responsive to said sensing means for deenergizing the power source and for selecting capacitors in the tank circuit to achieve a high L-C ratio; and

E. means for energizing said power source after the capacitors are selected to permit current to flow through the circuit until a further change in operating current occurs.

2. An RF heater as set forth in claim 1 wherein the oscillator is an electron tube-type oscillator.

3. An RF heater as set forth in claim 1 in which there are two capacitors in the tank circuit having different fixed values.

4. An RF heater as set forth in claim 1 in which the values of the capacitors are fixed and correspond to the frequency of the oscillator.

5. An RF heater as set forth in claim 1 wherein the work coil comprises a strip material in which the strip is helically wound in turns of increasing pitch and decreasing diameter from one end to the other.

6. An RF heater having an electron tube-type ocillator and a feedback system for switching capacitors into the tank circuit of the oscillator in response to changes in the materials to be heated comprising:

A. a power source; and

B. a control circuit including:

1 two capacitors and a load coil in the tank circuit of the oscillator,

2 means for sensing a change in plate current caused by the materials to be heated entering the load coil,

3 means responsive to said sensing means for deenergizing the power source and for selecting capacitors in the tank circuit to achieve a high L-C ratio thus increasing the circulating current in the tank circuit,

4 means for energizing the power source after the capacitors are selected to permit current to flow through the circuit until a further change in plate current occurs.

7. In an RF heater including an oscillator having a tank circuit and a heating coil, a control circuit for the heater comprising:

A. a power source;

B. at least two capacitors engagable in the tank circuit;

C. means for sensing a change in operating current in the circuit;

D. means responsive to said sensing means for deenergizing the power source and for selecting the capacitors to achieve a high L-C ratio thus increasing the circulating current in the tank circuit; and

E. means for energizing said power source after the capacitors are selected.

8. A control for a radio-frequency heater having an oscillator with a tank circuit and a load coil:

A. a plate transformer having a primary and a secondary;

B. means for sensing a change in plate current of the plate transformer including:

1 a current transformer having a primary connected to the power source,

2 means for rectifying the plate current, and

3 means for calibrating the rectified current;

C. means for de-energizing the plate transformer responsive to said sensing means including a first relay connected to the secondary of the plate transformer and operable by a change in plate current;

D. means for selecting capacitors in the tank circuit responsive to said sensing means including:

1 a first thermal relay connected to said first relay,

2 a switch relay connected to said thermal relay and to the tank circuit of the oscillator for switching a capacitor into and out of the tank circuit, and

E. means for energizing the plate transformer after the switch relay has been activated including a second thermal relay connected between said first relay and a third relay between said first relay and said plate transformer secondary, said second thermal relay having a time delay greater than that of said first thermal relay.

9. An RF heater comprising:

A. a power source;

B. an electron tube type oscillator having a tank circuit including at least two capacitors and a work coil, the work coil being a part of an oven comprisinga body of non-conductive material, a heat resistant layer within the body, a flat coil of conductive material helically wound about said body, a second body of non-conductive material surrounding said first body and spaced therefrom to provide an air space surrounding said second body which is impenetrable by RF rays;

C. means for sensing a change in operating current of the oscillator caused by a workpiece entering the coil;

D. means responsive tosaid sensing means for deenergizing the power source and for selecting capacitors in the tank circuit; and

E. means for energizing said power source after the capacitors are selected to permit current to flow through the circuit until a further change in operating current occurs. 

1. An RF heater comprising: A. a power source; B. an oscillator having a tank circuit including at least two capacitors and a work coil; C. means for sensing a change in operating current of the oscillator caused by a workpiece entering the coil; D. means responsive to said sensing means for de-energizing the power source and for selecting capacitors in the tank circuit to achieve a high L-C ratio; and E. means for energizing said power source after the capacitors are selected to permit current to flow through the circuit until a further change in operating current occurs.
 2. An RF heater as set forth in claim 1 wherein the oscillator is an electron tube-type oscillator.
 3. An RF heater as set forth in claim 1 in which there are two capacitors in the tank circuit having different fixed values.
 4. An RF heater as set forth in claim 1 in which the values of the capacitors are fixed and correspond to the frequency of the oscillator.
 5. An RF heater as set forth in claim 1 wherein the work coil comprises a strip material in which the strip is helically wound in turns of increasing pitch and decreasing diameter from one end to the other.
 6. An RF heater having an electron tube-type ocillator and a feedback system for switching capacitors into the tank circuit of the oscillator in response to changes in the materials to be heated comprising: A. a power source; and B. a control circuit including: 1 two capacitors and a load coil in the tank circuit of the oscillator, 2 means for sensing a change in plate current caused by the materials to be heated entering the load coil, 3 means responsive to said sensing means for de-energizing the power source and for selecting capacitors in the tank circuit to achieve a high L-C ratio thus increasing the circulating current in the tank circuit, 4 means for energizing the power source after the capacitors are selected to permit current to flow through the circuit until a further change in plate current occurs.
 7. In an RF heater including an oscillator having a tank circuit and a heating coil, a control circuit for the heater comprising: A. a power source; B. at least two capacitors engageable in the tank circuit; C. means for sensing a change in operating current in the circuit; D. means responsive to said sensing means for de-energizing the power source and for selecting the capacitors to achieve a high L-C ratio thus increasing the circulating current in the tank circuit; and E. means for energizing said power source after the capacitors are selected.
 7. In an RF heater including an oscillator having a tank circuit and a heating coil, a control circuit for the heater comprising: A. a power source; B. at least two capacitors engageable in the tank circuit; C. means for sensing a change in operating current in the circuit; D. means responsive to said sensing means for de-energizing the power source and for selecting the capacitors to achieve a high L-C ratio thus increasing the circulating current in the tank circuit; and E. means for energizing said power source after the capacitors are selected.
 8. A control for a radio-frequency heater having an oscillator with a tank circuit and a load coil: A. a plate transformer having a primary and a secondary; B. means for sensing a change in plate current of the plate transformer including: 1 a current transformer having a primary connected to the power source, 2 means for rectifying the plate current, and 3 means for calibrating the rectified current; C. means for de-energizing the plate transformer responsive to said sensing means including a first relay connected to the secondary of the plate transformer and operable by a change in plate current; D. means for selecting capacitors in the tank circuit responsive to said sensing means including: 1 a first thermal relay connected to said first relay, 2 a switch relay connected to said thermal relay and to the tank circuit of the oscillator for switching a capacitor into and out of the tank circuit, and E. means for energizing the plate transformer after the switch relay has been activated including a second thermal relay connected between said first relay and a third relay between said first relay and said plate transformer secondary, said second thermal relay having a time delay greater than that of said first thermal relay. 